FIG. 1 is a simplified side cross-sectional view of a portion of a field emission display 10 including a faceplate 20 and a baseplate 21 in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate 20 includes a transparent viewing screen 22, a transparent conductive layer 24 and a cathodoluminescent layer 26. The transparent viewing screen 22 supports the layers 24 and 26, acts as a viewing surface and as a wall for a hermetically sealed package formed between the viewing screen 22 and the baseplate 21. The viewing screen 22 may be formed from glass. The transparent conductive layer 24 may be formed from indium tin oxide. The cathodoluminescent layer 26 may be segmented into pixels yielding different colors for color displays. Materials useful as cathodoluminescent materials in the cathodoluminescent layer 26 include Y.sub.2 O.sub.3 :Eu (red, phosphor P-56), Y.sub.3 (Al, Ga).sub.5 O.sub.12 :Tb (green, phosphor P-53) and Y.sub.2 (SiO.sub.5):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda Pa. or from Nichia of Japan.
The baseplate 21 includes emitters 30 formed on a planar surface of a substrate 32. The substrate 32 is coated with a dielectric layer 34. In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer 34 is formed to have a thickness that is less than a height of the emitters 30. This thickness is on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid 38 is formed on the dielectric layer 34. The extraction grid 38 may be formed, for example, as a thin layer of doped polysilicon. The radius of an opening 40 created in the extraction grid 38, which is also approximately the separation of the extraction grid 38 from the tip of the emitter 30, is about 0.4 microns, although larger or smaller openings 40 may also be employed.
The baseplate 21 also includes a field effect transistor ("FET") 50 formed in the surface of the substrate 32 for controlling the supply of electrons to the emitter 30. The FET 50 includes an n-tank 52 formed in the surface of the substrate 32 beneath the emitter 30. The n-tank 52 serves as a drain for the FET 50 and may be formed via conventional masking and ion implantation processes. The FET 50 also includes a source 54 and a gate electrode 56. The gate electrode 56 is separated from the substrate 32 by a gate oxide 57 and a field oxide layer 58. The emitter 30 is typically about a micron tall, and several emitters 30 are generally included together with each n-tank 52, although only one emitter 30 is illustrated.
The substrate 32 may be formed from p-type silicon material having an acceptor concentration N.sub.A ca. 1-5.times.10.sup.15 /cm.sup.3, while the n-tank 52 may have a surface donor concentration N.sub.D ca. 1-2.times.10.sup.16 /cm.sup.3.
In operation, the extraction grid 38 is biased to a voltage on the order of 40-80 volts, although higher or lower voltages may be used, while the substrate 32 is maintained at a voltage of about zero volts. Signals coupled to the gate 56 of the FET 50 turn the FET 50 on, allowing electrons to flow from the source 54 to the n-tank 52 and thus to the emitter 30. Intense electrical fields between the emitter 30 and the extraction grid 38 then cause field emission of electrons from the emitter 30. A larger positive voltage, ranging up to as much as 5,000 volts or more but often 2,500 volts or less, is applied to the faceplate 20 via the transparent conductive layer 24. The electrons emitted from the emitter 30 are accelerated to the faceplate 20 by this voltage and strike the cathodoluminescent layer 26. This causes light emission in selected areas, ie., those areas adjacent to where the FETs 50 are conducting, and forms luminous images such as text, pictures and the like. Integrating the FETs 50 in the substrate 32 to provide an active display 10 (i.e., a display 10 including active circuitry for addressing and providing control signals to specific emitters 30, etc.) yields advantages in size, simplicity and ease of interconnection of the display 10 to other electronic componentry.
When the emitted electrons strike the cathodoluminescent layer 26, compounds in the cathodoluminescent layer 26 dissociate. This causes outgassing of materials from the cathodoluminescent layer 26. When the outgassed materials react with the emitters 30, a barrier height of the emitters 30 may increase. When the emitter barrier height increases, the emitted current is reduced. This reduces the luminance of the display 10.
Residual gas analysis indicates that the dominant materials outgassed from some display cathodoluminescent layers 26 include oxygen and hydroxyl radicals. This leads to oxidation of the emitters 30 and especially emitters 30 formed from silicon. Silicon emitters 30 are useful because they are readily formed and integrated with other electronic devices on silicon substrates. Electron emission is reduced when silicon emitters 30 oxidize. This degrades performance of the display 10.
Therefore there is a need for a way to prevent degradation, and especially oxidation, of emitters 30 used in displays 10.